Add grpc server and client metrics to Teleport

[rcanderson23]

This is part of https://github.com/gravitational/cloud/issues/1468

This add grpc client metrics to the Proxy service and server metrics to the Auth service via a grpc interceptor. By default these metrics are lazy loaded and latency is disabled by default. The latency metrics are turned off by default as they are high cardinality. Users can enable them enabling the metrics_service using the following stanza:

metrics_service:
  enabled: yes
  listen_addr: 0.0.0.0:9090
  optional_metrics:
    grpc_server_latency: true
    grpc_client_latency: true

I went back and forth on having these in their respective service stanzas, it might be more clear if they configured in their respective service and will change if feedback agrees.

Documentation is missing from this PR which I will add once I have feedback on rest to avoid having to make the changes twice.

Mock up dashboard with these can be seen here (still WIP)

[rosstimothy]

LGTM. Could you add the new Metrics config items to TestConfigReading before merging?

[rosstimothy]

If this wasn't a pointer you could rely on default values to do the right thing at the moment since Metrics only contains one boolean. If Metrics was to be extended in the future then relying on default values wouldn't work so maybe it's fine to leave as it is.